The invention is directed to a single mode optical waveguide fiber designed to manage both polarization mode dispersion (PMD) and total dispersion (TD).
In previous applications, Ser. No. 08/423,656 (abandoned) and Ser. No. 08/353,822 solutions to the problems of managing total dispersion and polarization mode dispersion in optical waveguide fibers have been discussed in detail.
Total dispersion is managed by causing the total dispersion to alternate between positive and negative values, thereby producing a net algebraic sum of zero for the products, length times total dispersion for that length. In general the TD can be managed in this way over essentially any pre-selected range of light wavelengths. A wavelength range of particular interest is that in the range of about 1490 nm to 1650 nm. A typical silica based waveguide exhibits a low attenuation over this range.
The dispersion times length products are summed over the entire waveguide length. Total dispersion may be caused to alternate between positive and negative values by introducing geometrical or refractive index perturbations into the waveguide core.
Polarization mode dispersion is managed by transferring power between the two polarization modes, i.e., mixing the modes, thereby effectively limiting or eliminating the difference in travel time between the two modes. The birefringence axes in the waveguide are made to change relative orientation by 90.degree. periodically along the fiber length. The birefringence is managed to be a net of substantially zero for the total waveguide fiber length. That is, the alternating birefringence provides a homogeneous optical path length for the two polarization modes of light launched into the waveguide fiber.
Here again, a method for providing alternating birefringence axes includes introducing geometrical or index perturbations into the waveguide core.
The two dispersion types can be managed in the same fiber because:
perturbations large enough to produce a change in birefringence are still small compared to the perturbations required to change the sign of the total dispersion; and, PA1 the requirements on spacing of perturbations for management of total dispersion can be made compatible with the spacing of perturbations required for polarization mode mixing. PA1 the very highest bit rate systems; PA1 systems using wavelength division multiplexing; or, PA1 systems using long regenerator spacing, with or without optical amplifiers. PA1 fabricating a core preform by any of several methods known in the art, including inside and outside vapor deposition and axial vapor deposition; PA1 forming a first pattern of perturbations in the preform surface to mix the polarization modes; PA1 forming a second pattern of perturbations in the preform surface to cause the sign of the total dispersion to alternate between positive and negative values, thereby controlling the total dispersion to a pre-selected value; PA1 applying a clad layer about the core preform in such a way that the glass draw preform has a uniform cylindrical surface; and, PA1 drawing the draw preform into a waveguide fiber having a substantially uniform cylindrical surface, thereby impressing the perturbations into the waveguide core surface.
Hence, it is possible to substantially eliminate polarization mode dispersion and total dispersion in an optical waveguide fiber. In addition, the operating or signal wavelength may be made different from the zero dispersion wavelength, to avoid four photon mixing, even though the zero dispersion wavelength may be changed from segment to segment within a waveguide. Finally, the perturbations which provide the desired management of dispersion are such that waveguide attenuation is not adversely affected.
An additional benefit is noted in a preferred embodiment below wherein the perturbations which control total dispersion are decoupled from those which mix the polarization to limit polarization mode dispersion.
The invention disclosed in this application is thus an extremely low dispersion, low attenuation waveguide designed for: